Wtf did Mariam take vampire’s soul? America I am confusion?????????

Because he was afraid that vampire boy would leave and drink another person's blood and to stop him he took his soul as a bargaining chip ╮(￣▽￣)╭
So now if Bok Hee 'leaves', Mariam can 'blackmail' him not to leave.
(But of course that should never happen)

Yeah, that's was a bit anticlimatic. I thought Bok loved him, that's enough reason to stay together. Also, vampires don't have illimited souls, so Mariam would have to feed from other humans as well at some point. Makes no sense in my head.